This invention relates to a gas turbine combustor and combustion method, and, more particularly, to a gas turbine combustor and combustion method wherein reducing substances for NOx are produced through combustion of fuel supplied to the combustor and NOx produced by premixed combustion is reduced with the reducing substances thereby enabling a reduction in NOx emission of the combustor.
Nitrogen oxides (referred to as NOx) produced by combustion in a gas turbine combustor and called thermal NOx is caused by nitrogen in air. A mechanism for producing the thermal NOx is explained by a Zerdovich mechanism. That is, this is caused by the following elementary reaction. EQU N.sub.2 +O.sub.2 .fwdarw.NO+N (1) EQU N+O.sub.2 .fwdarw.NO+O (2) EQU N+OH.fwdarw.NO+H (3)
It can be understood from these reactions that generation of nitrogen atoms by a dissociation reaction of nitrogen molecules in the equation (1) is an initiation reaction. More specifically, nitrogen atoms N produced in the equation (1) are oxidized by oxygen molecules and hydroxyl radicals in the equations (2), (3), respectively, to becomes NO. A series of these reactions increase as a flame temperature increases, whereby an amount of thermal NOx produced increases.
Therefore, the flame temperature must not be increased in order to reduce the production of thermal NOx. At present, a lean-combustion system described below is primarily employed as a low NOx combustion system using this principle, although there is available a system for supplying steam or combustion gas into a combustor or the like.
The lean-combustion system cools a flame temperature with a large amount of air to prevent temperature increase. More specifically, a combustor is controlled so as not to have any area wherein the air ratio, that is, the ratio of an amount of really supplied air to an amount of air necessary for perfect combustion of the supplied fuel or, in other words, the ratio of actual air supplied to the theoretical air required for stoichiometric combustion, is near 1.0 thereby effecting combustion with a high air-ratio of about 2.0 in all of the areas.
Japanese Utility Model Laid-Open No. 57-154853 relating to a lean-combustion system discloses a system for effecting lean-combustion as stably as possible by supplying air into a combustor when an air pressure is low by use of a pressure in a gas turbine casing. Japanese Utility Model Laid-Open No. 57-150373 discloses an air introducing device for a combustor of a gas turbine.
The lean-combustion system for preventing an increase in a combustion temperature by supplying a large amount of premixed air and fuel for forming a high air-ratio premixed combustion flame (flame produced when premixed fuel and air are combusted) has a problem of blow-off. This is because a premixed combustion flame is generally most stable when an air ratio is in the vicinity of 1 and blow-off is liable to arise when the air ratio is greater than 1.
Japanese Patent Laid-Open No. 61-41810 discloses a system wherein fuel is separately supplied into two first and second regions in a combustor, with the fuel in the first region being combusted at a high air-ratio of about 1.2 for perfect combustion, and then NOx produced there is reduced by a low air-ratio combustion flame in the second region including a small amount of oxygen and a large amount of reducing combustible gas. The combustible gas remaining in the second region is oxidized and combusted by air from an after-air port in a rear flow for thereby decreasing an amount of NOx.
Japanese Patent Laid-Open No. 61-41810 does not describe whether the flame in the first region is a premixed combustion flame or a diffusion combustion flame and if the flame is the premixed flame, a problem of blow-off arises.
In addition, in the last-mentioned Patent Laid-Open, a flame interference should occur between a high air-ratio combustion flame and low air-ratio combustion flame, judging a relationship between the locations where both flames are formed from an embodiment disclosed in the Patent Laid-Open. More specifically, the positional relationship of the flames is such that excessive oxygen is diffused from the side of the high air-ratio combustion flame to the side of the low air-ratio combustion flame, while fuel is diffused from the side of the low air-ratio flame. Because of the flame interference, NOx produced from the high air-ratio combustion flame is not effectively reduced by the low air-ratio combustion flame. As a result, there is a problem that the respective flames cannot achieve their roles sufficiently.